Polycarbonate polymers are excellent molding materials as products made therefrom have high impact strength, toughness, high transparency, wide temperature limits (high impact resistance below -60.degree. C. and a UL thermal endurance rating of 115.degree. C. with impact), good dimensional stability, high creep resistance and electrical properties which qualify it as sole support for current carrying parts.
Polycarbonates are, however, very difficult to fabricate from melts for the reason that the high temperatures required lead to thermal and oxidative degradation. This is especially aggravated in pigmented, e.g., titanium dioxide--containing, modifications. Degradation results in decreases in melt viscosity and development of undesirable color. Although phosphites alone appear to have an advantageous stabilizing effect, in general, higher than desirable amounts are necessary. Furthermore, it is known from U.S. Pat. Nos. 3,087,908, and 2,999,835 that polycarbonates can be admixed with polydiorganosiloxanes to produce melt processed materials which are free from blemishes, undesirable color effects, bubbles and craters. However, there is no suggestion that thermal oxidative stability can be improved by this expedient, especially when pigments are also present.
It has now been discovered that minor effective amounts of chain-stopped polyorganohydrogensiloxanes are remarkably effective per se and in combination with organic phosphites and phosphonites and/or epoxides to improve the stability of polycarbonate resins. In contrast to the additives of Caird, incorporated herein by reference, the superiority of those herein is believed to be due to the unique presence of silicon bonded active hydrogen--which confers anti-oxidant-reducing agent-thermal stabilizing properties in the additive.